The complexity of semiconductor chips has increased dramatically over the past several years. Such increased complexity has lead to an increase in the number of input and output leads or contacts required for each chip package. Further, with this increased complexity and the constant need to shorten chip production times, methods and systems for more rapid and accurate inspection of chip packages are needed.
Three dimensional laser beam sensor systems utilizing laser optical triangulation have been used to inspect chip packages. Such a system consists of a semiconductor diode laser, a beam deflector (for example, an acousto-optical (AO) deflector, also called an AO modulator) and a position sensitive device (PSD). The laser diode provides the light source for measurements. The beam deflector directs the laser beam to "sweep" the target object along a "sweep" direction. It will be observed that the terms "sweep" and "scan" may be used interchangeably. However, "sweep" will be used generally herein to refer to the specific "scanning" produced by a beam deflector, or AO deflector. Accordingly, to cover the entire target area, such systems typically rely on a mechanical scan, or translation, of the sensor system, or equivalently, the target object, in a direction perpendicular to the AO sweep direction. The PSD measures the height of the target object at each scan point and the data are stored until an image record for the entire object or a selected portion is collected. The stored image record may then be compared to a manufacturer's specification for the object or a selected portion to determine whether the object meets specification.
U.S. Pat. No. 5,554,858 issued to Costa et al. (the "'858 patent"), expressly incorporated herein by reference, describes one such system. A laser light source combined with an AO deflector is positioned to illuminate an object and sweep along the AO deflection direction while commercially available linear motion tables provide the transverse scanning translation. PSD sensors are positioned on both sides of the incident beam to receive light reflected from the sample and imaged into the PSDs by lenses. Further, the '858 patent describes use of multi-channel PSDs to collect the imaging data. A PSD provides an analog output current ratio proportional to the position of a light spot falling along its length. A multi-channel PSD has a segmented photo-sensitive area, the multiple segments comprising the multiple data channels. When used with a selective sampling technique, the multi-channel PSD can reduce the effect of stray or multipli-reflected light.
U.S. Pat. No. 5,859,924 to Liu et al. (the "'924 patent"), expressly incorporated herein by reference, describes another such system. The '924 patent describes another scanning system using optical triangulation techniques. This system uses a laser beam and AO deflector and one or more photosensitive devices to collect light reflected off-axially (from the axis of the incident light source) from the target object. Further, the system uses a polarizing beam splitter in the optical path of the incident beam to direct light reflected co-axially from the target object into a photo diode array for intensity measurement.
In conventional three-dimensional scanning systems such as the aforementioned, the AO deflecting swath is limited by the finite bandwidth of the AO device. An AO device deflects an optical beam by having an ultrasonic sound wave applied by piezo-electric signal transducers to a suitable crystalline material. The resultant sound wave in the crystal produces a periodic variation in the crystal's index of refraction which is used to diffract a fraction of an incident beam of monochromatic light. This fraction of the incident beam of monochromatic light is called the first order diffracted output. If no RF signal is applied, only the zero order non-diffracted incident beam will exit from the AO device.
The angle of diffraction .theta. of the first order output from the incident beam direction is given approximately by: ##EQU1##
where .lambda. is the wavelength of the incident beam measured in air, f is the ultrasonic frequency of the sound wave, and V is the ultrasonic velocity of the sound wave. Those skilled in the art will appreciate from equation (1) that the diffraction angle .theta. varies directly with the ultrasonic frequency f. Therefore, since the process is linear, an AO diffraction angle range or "sweep angle" range, .DELTA..theta., is proportional to a change in the ultrasonic frequency, .DELTA.f, according to: ##EQU2##
The first order output can be quickly "swept" through the AO sweep angle range by continuously changing the ultrasonic drive frequency to the AO deflector. Equation (2) shows that the maximum AO sweep angle range is limited by the bandwidth, or range of frequencies, which can be used with the AO deflector. Thus, the smaller the AO bandwidth, the smaller the concomitant AO sweep angle range and the more mechanical scans required to complete a given area measurement. For example, to cover a 12".times.8" ball grid array tray or an 8" flip chip wafer with an AO deflector scanning with a single beam, several hundred parallel mechanical scans might be required.
Other beam deflectors such as rotating polygonal scanners or galvanometer mirrors can also sweep a beam through a finite angle in a finite time. The speed limitations of these devices similarly impose a speed constraint on the operation of any scanning system in which they function.
Further, the speed of a system using a conventional PSD will be limited by the design and physical parameters of the PSD and its associated electronics. A current state-of-the-art PSD may have rise and fall times of a few hundred nanoseconds each. Therefore, the maximum throughput from a real time 3D sensor is limited by the bandwidths of both the AO deflector and the PSD.